Carbon powder for catalyst, catalyst, electrode catalyst layer, membrane electrode assembly, and fuel cell using the carbon powder

ABSTRACT

The present invention is to provide a carbon powder that can provide a catalyst having excellent durability and a catalyst. A carbon powder for catalyst of the present invention is a carbon powder containing as a main component carbon, which has a BET specific surface area per unit weight of 900 m 2 /g or greater, and a ratio R′ (D′/G intensity ratio) of peak intensity for a D′-band (D′ intensity) measured in the vicinity of 1620 cm −1  to peak intensity for a G-band (G intensity) measured in the vicinity of 1580 cm −1  by Raman spectroscopy of 0.6 or less.

TECHNICAL FIELD

The present invention relates to a carbon powder for catalyst, and moreparticularly, to a carbon powder for catalyst used in a fuel cell, acatalyst using the carbon powder for catalyst, an electrode catalystlayer, a membrane electrode assembly, and a fuel cell.

BACKGROUND ART

A polymer electrolyte fuel cell using a proton conductive solid polymermembrane operates at a low temperature in comparison to other types offuel cells, for example, a solid oxide fuel cell or a molten carbonatefuel cell. For this reason, the polymer electrolyte fuel cell has beenexpected to be used as a power source for energy storage system or adriving power source for a vehicle such as a car, and practical usesthereof have been started.

In general, expensive metal catalysts such as Pt (platinum) and Ptalloys have been used for such a polymer electrolyte fuel cell. Alsographitized carbon has been used as a support for supporting a metalcatalyst in terms of water repellency and corrosion resistance. Forexample, Patent Literature 1 discloses a technique using a supporthaving an average lattice spacing of (002) plane, d₀₀₂, of 0.338 to0.355 nm, a specific surface area of 80 to 250 m²/g, and a bulk densityof 0.30 to 0.45 g/ml. Patent Literature 1 discloses that durability canbe improved by using the graphitized support.

CITATION LIST Patent Literature

Patent Literature 1: JP 2005-26174 A

SUMMARY OF INVENTION Problem to be Solved by the Invention

In the support disclosed in the Patent Literature 1, however, since thesupport has excellent durability but small specific surface area, therehas been a problem in terms of decreased activity over time.

Accordingly, the present invention has been made in light of theaforementioned circumstances and aims at providing a carbon powder forcatalyst which can suppress the decrease in catalytic activity whilemaintaining durability of support.

Another object of the present invention is to provide a catalyst, anelectrode catalyst layer, a membrane electrode assembly, and a fuel cellwhich are excellent in durability and power generation performance.

Means to Solve the Problem

The present inventors have intensively studied to solve theaforementioned problems, to find that the problems can be solved byusing as a support a carbon powder for catalyst having a specifiedspecific surface area and a D/G intensity ratio, and eventually thepresent invention has been completed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating a basicconfiguration of a polymer electrolyte fuel cell according to anembodiment of the present invention. In FIG. 1, reference sign 1represents a polymer electrolyte fuel cell (PEFC); reference sign 2represents a solid polymer electrolyte membrane; reference sign 3 arepresents an anode catalyst layer; reference sign 3 c represents acathode catalyst layer; reference sign 4 a represents an anode gasdiffusion layer; reference sign 4 c represents a cathode gas diffusionlayer; reference sign 5 a represents an anode separator; reference sign5 c represents a cathode separator; reference sign 6 a represents ananode gas passage; reference sign 6 c represents a cathode gas passage;reference sign 7 represents a coolant passage; and reference sign 10represents a membrane electrode assembly (MEA).

FIG. 2 is a schematic cross-sectional diagram illustrating a shape and astructure of a catalyst according to an embodiment of the presentinvention. In FIG. 2, reference sign 20 represents a catalyst; referencesign 22 represents a catalyst metal; reference sign 23 represents asupport; reference sign 24 represents a mesopore; and reference sign 25represents a micropore.

FIG. 3 is a schematic diagram illustrating as an example a relationbetween a catalyst and an electrolyte in a catalyst layer when a carbonpowder is as illustrated in FIG. 2. In FIG. 3, reference sign 22represents a catalyst metal; reference sign 23 represents a support;reference sign 24 represents a mesopore; reference sign 25 represents amicropore; and reference sign 26 represents an electrolyte.

FIG. 4 is a graph illustrating an evaluation result of platinum coatingof supports A and B produced in Examples 1 and 2 and a support Eproduced in Comparative Example 3, in Experiment 1.

FIG. 5 is a graph illustrating an evaluation result of power generationperformance of MEA (1) produced in Example 6 and MEA (2) produced inComparative Example 7, in Experiment 3.

DESCRIPTION OF EMBODIMENTS

A carbon powder for catalyst (simply also referred to as a “carbonpowder” in this description) of the present invention contains carbon asa main component. As used herein, the phrase “contain(s) carbon as amain component” includes both “consist(s) only of carbon” and“consist(s) substantially of carbon”, and an element other than carbonmay be incorporated. The phrase “consist(s) substantially of carbon”means that 80 wt % or more, preferably 95 wt % or more (upper limit:less than 100 wt %) of the entire carbon power is composed of carbon.

The carbon powder for catalyst of the present invention satisfies thefollowing configurations (a) and (b):

(a) a BET specific surface area per unit weight is 900 m²/g or greater;and

(b) a ratio R′ (D′/G intensity ratio) of peak intensity for a D′-band(D′ intensity) measured in the vicinity of 1620 cm⁻¹ to peak intensityfor a G-band (G intensity) measured in the vicinity of 1580 cm⁻¹ byRaman spectroscopy is 0.6 or less. In this description, a G-bandmeasured in the vicinity of 1580 cm⁻¹ by the Raman spectroscopy is alsosimply referred to as a “G-band”. In this description, a D′-bandmeasured in the vicinity of 1620 cm⁻¹ by the Raman spectroscopy is alsosimply referred to as a “D′-band”. In addition, peak intensity forG-band and D′-band is also simply referred to as “G intensity” and “D′intensity”, respectively. Moreover, a ratio of D′ intensity to Gintensity is also simply referred to as an “R′ value” or a “D′/Gintensity ratio”. In the carbon powder for catalyst having the aboveconfiguration, a specific surface area is large and an edge quantityserving as a starting point of electrochemical corrosion is small. Forthis reason, by using the carbon powder for catalyst of the presentinvention as a support, a catalyst that is excellent in durability andcan maintain catalytic activity can be provided.

The support disclosed in the Patent Literature 1 can be obtained bygraphitization of carbon particles through heat treatment at 2000 to3000° C. (see paragraph [0016]). The support disclosed in the PatentLiterature 1 can improve its durability by the graphitization treatment.However, since the support has a small specific surface area as of 250m²/g or less, a coating ratio of a catalyst metal (for example,platinum) with an electrolyte in the formation of an electrode catalystlayer is high. For this reason, gas transport ability of the electrodecatalyst layer is decreased, to lower activity.

In contrast, the carbon powder according to the present inventionsatisfies the configuration (a) described above. Since the carbon powderhas a sufficient specific surface area due to the configuration (a),electrical double layer capacitance is large. In addition, the carbonpowder according to the present invention satisfies the configuration(b) described above. The G-band is a peak resulting from graphite(oscillation in a hexagonal lattice of a carbon atom) observed in thevicinity of 1580 cm⁻¹ by Raman scattering analysis. The D′-band isobserved as a shoulder of the G-band in the vicinity of 1620 cm⁻¹ byRaman scattering analysis. This D′-band results from disturbance(disorder) or defect of a graphite structure, and appears when a crystalsize of graphite is small or numerous edges of graphene sheet exist. Anelectronic state of the edge (end) of a graphene molecule is likely tobe a starting point of carbon corrosion, unlike a center (six-memberedring) of a graphene molecule. That is, a small R′ value indicates thatan edge quantity of carbon (graphene) existing in a graphite structureand serving as a starting point of electrochemical corrosion is small.Therefore, durability can be improved due to the above configuration(b), and thus deterioration in catalytic activity can be effectivelysuppressed and prevented.

In addition, the carbon powder according to the present inventionpreferably has a configuration (c) that a ratio R (D/G intensity ratio)of peak intensity (D intensity) for a D-band measured in the vicinity of1360 cm⁻¹ to G intensity is 1.7 or more. In this description, a D-bandobserved in the vicinity of 1360 cm⁻¹ by Raman spectroscopy is alsosimply referred to as a “D-band”. In addition, peak intensity for theD-band is also referred to as “D intensity”. Moreover, a ratio of Dintensity to G intensity is also simply referred to as an “R value” or a“D/G intensity ratio”. Herein, the D-band is observed in the vicinity of1360 cm⁻¹ by Raman scattering analysis, results from disturbance(disorder) or defect of a graphite structure, and appears whenorientation of graphene molecules is high or a graphitization degree ishigh. That is, a large R value indicates that a graphitization degree ofa carbon powder (support) is low. Therefore, by the above configuration(c), electrical double layer capacitance per surface area of the carbonpowder increases, to improve catalytic activity more effectively.

The G-band, D′-band, and D-band, and peak intensities for these bandshave been well known in the art. For example, see R. Vidano and D. BFischbach, J. Am. Ceram. Soc. 61 (1978) 13-17 or G. Katagiri, H. Ishidaand A. Ishitani, Carbon 26 (1988) 565-571.

Therefore, the carbon powder for catalyst of the present invention hasexcellent durability and can exhibit and maintain high catalyticactivity when a catalyst metal is supported thereon. For this reason,the carbon powder for catalyst of the present invention can be suitablyused for a catalyst, particularly, as a support of a catalyst used forfuel cell. That is, the present invention involves a catalyst comprisinga catalyst metal supported on the carbon powder for catalyst of thepresent invention. The carbon powder for catalyst (support) of thepresent invention has a high specific surface area. Therefore, accordingto the catalyst of the present invention, dispersability of the catalystcan be improved, to increase an electrochemical reaction area, i.e., toimprove power generation performance. In addition, the carbon powder forcatalyst (support) of the present invention has a small carbon edgequantity. Therefore, according to the catalyst of the present invention,performance degradation due to carbon corrosion can be suppressed andprevented, that is, durability can be improved. A catalyst having acatalyst metal supported on the carbon powder for catalyst of thepresent invention can have excellent durability, exhibit and maintainhigh catalytic activity (to facilitate catalyst reaction). Thus, amembrane electrode assembly and a fuel cell having a catalyst layerusing such a catalyst have excellent power generation performance anddurability. Accordingly, the present invention provides an electrodecatalyst layer for fuel cell including the catalyst and an electrolyte,a membrane electrode assembly for fuel cell including the electrodecatalyst layer for fuel cell, and a fuel cell including the membraneelectrode assembly for fuel cell.

Hereinafter, embodiments of a catalyst according to the presentinvention and embodiments of a catalyst layer, and a membrane electrodeassembly (MEA) and a fuel cell using the catalyst will be described indetail appropriately with reference to the drawings. However, thepresent invention is not limited to the following embodiments. Inaddition, figures may be expressed in an exaggerated manner for theconvenience of description, and in the figures, scaling factors ofcomponents may be different from actual values thereof. In addition, inthe description of the embodiments of the present invention withreference to the drawings, the same components are denoted by the samereference numerals, and redundant description is omitted.

In this description, “X to Y” representing a range denotes “X or moreand Y or less”. Unless otherwise noted, operation and the measurement ofphysical properties are performed at a room temperature (20 to 25° C.)and a relative humidity of 40 to 50%.

[Fuel Cell]

A fuel cell comprises a membrane electrode assembly (MEA) and a pair ofseparators including an anode-side separator having a fuel gas passagethrough which a fuel gas flows and a cathode-side separator having anoxidant gas passage through which an oxidant gas flows. The fuel cellaccording to the present embodiment has excellent durability and canexhibit a high power generation performance.

FIG. 1 is a schematic diagram illustrating a basic configuration of apolymer electrolyte fuel cell (PEFC) 1 according to an embodiment of thepresent invention. First, a PEFC 1 is configured to comprise a solidpolymer electrolyte membrane 2 and a pair of catalyst layers (anodecatalyst layer 3 a and cathode catalyst layer 3 c) interposing the solidpolymer electrolyte membrane 2. A stacked body of the solid polymerelectrolyte membrane 2 and the catalyst layers (3 a, 3 c) is sandwichedby a pair of gas diffusion layers (GDLs) (anode gas diffusion layer 4 aand cathode gas diffusion layer 4 c). In this manner, the solid polymerelectrolyte membrane 2, a pair of the catalyst layers (3 a, 3 c), and apair of gas diffusion layers (4 a, 4 c) in the stacked state constitutea membrane electrode assembly (MEA) 10.

In the PEFC 1, the MEA 10 is sandwiched by a pair of separators (anodeseparator 5 a and cathode separator 5 c). In FIG. 1, the separators (5a, 5 c) are illustrated to be positioned at two ends of the MEA 10illustrated. In general, in a fuel cell stack where a plurality of MEAsare stacked, the separator is also used as a separator for adjacent PEFC(not shown). In other words, MEAs in a fuel cell stack are sequentiallystacked through the separator to constitute the stack. In an actual fuelcell stack, a gas sealing member is disposed between the separators (5a, 5 c) and the solid polymer electrolyte membrane 2 and between thePEFC 1 and a different PEFC adjacent thereto. However, it is omitted inFIG. 1.

The separators (5 a, 5 c) are obtained by applying a pressing process toa thin board having a thickness of, for example, 0.5 mm or less to forma corrugating shape illustrated in FIG. 1. Convex portions of theseparators 5 a and 5 c seen from the MEA side are in contact with theMEA 10. This secures an electrical connection with the MEA 10. Concaveportions (spaces between the separator and the MEA formed by thecorrugating shapes of the separators) of the separators (5 a and 5 c)seen from the MEA side function as a gas passage for passing a gasduring the operation of the PEFC 1. Specifically, a fuel gas (forexample, hydrogen) flows through a gas passage 6 a of the anodeseparator 5 a, and an oxidant gas (for example, air) flows through a gaspassage 6 c of the cathode separator 5 c.

On the other hand, concave portions of the separators (5 a, 5 c) seenfrom the side opposite to the MEA side function as a coolant passage 7for passing a coolant (e.g. water) for cooling the PEFC during theoperation of the PEFC 1. In addition, manifolds (not shown) aretypically installed in the separators. The manifold functions as aconnecting means for connecting cells when the stack is configured.According to the configuration, a mechanical strength of the fuel cellstack can be secured.

In the embodiment illustrated in FIG. 1, each of the separators (5 a, 5c) is formed in a corrugating shape. However, the separator is notlimited to such a corrugating shape. If it can serve as a gas passageand a coolant passage, arbitrary shape such as a flat shape and apartially corrugating shape may be employed.

The fuel cell including the MEA according to the present invention asdescribed above has excellent performance of power generation anddurability. Herein, the type of the fuel cell is not particularlylimited. In the above description, the polymer electrolyte fuel cell isexemplified, but besides, an alkali fuel cell, a direct methanol fuelcell, a micro fuel cell, and the like may be exemplified. Among the fuelcells, due to a small size and capability of obtaining high density andhigh power, a polymer electrolyte fuel cell (PEFC) is preferred. Inaddition, the fuel cell is useful as a power source for energy storagesystem besides a power source for a vehicle such as a car where amounting space is limited. Among the power sources, the fuel cell isparticularly preferably used as a power source for a vehicle such as acar where a high output voltage is required after the stopping ofoperation for a relatively long time.

A fuel used for operating the fuel cell is not particularly limited. Forexample, hydrogen, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol,secondary butanol, tertiary butanol, dimethyl ether, diethyl ether,ethylene glycol, diethylene glycol, or the like can be used. Among them,in view of capability of high output, hydrogen or methanol is preferablyused.

In addition, although application use of the fuel cell is notparticularly limited, the fuel cell is preferably applied to vehicles.The electrolyte membrane-electrode assembly according to the presentinvention has excellent power generation performance and durability, andcan be downsized. Therefore, in terms of mountability on a vehicle, thefuel cell according to the present invention is particularlyadvantageous in the case where the fuel cell is applied to a vehicle.

Hereinafter, members constituting the fuel cell according to the presentinvention will be described in brief, but the scope of the presentinvention is not limited only to the following forms.

[Catalyst (Electrode Catalyst)]

A catalyst (electrode catalyst) is composed of a carbon powder (support)and a catalyst metal supported on the carbon powder. The carbon powder(support) satisfies the following configurations (a) and (b):

(a) it has a BET specific surface area per unit weight of 900 m²/g orgreater; and

(b) it has a ratio R′ (D′/G intensity ratio) of D′ intensity to Gintensity of 0.6 or less.

Since the carbon powder has a sufficient specific surface area due tothe configuration (a), large electrical double layer capacitance can beattained. For this reason, a catalyst having a catalyst metal supportedon the carbon powder satisfying the configuration (a) can exhibit highactivity. On the other hand, when a BET specific surface area per unitweight of a carbon powder is less than 900 m²/g, a coating ratio of acatalyst with an electrolyte in the formation of an electrode catalystlayer by using a catalyst having a catalyst metal supported on thecarbon powder increases due to its small specific surface area. For thisreason, gas transport ability in the electrode catalyst layer is loweredto decrease activity. In view of further improvement of electricaldouble layer capacitance, the BET specific surface area of the carbonpowder is preferably 1000 to 3000 m²/g, and more preferably 1100 to 1800m²/g.

In this description, the “BET specific surface area (m²/g support)” ismeasured by a nitrogen adsorption method. Specifically, about 0.04 to0.07 g of a sample (carbon powder or catalyst powder) is accuratelyweighed and sealed in a sample tube. The sample tube is preliminarilydried in a vacuum drier at 90° C. for several hours, to obtain a samplefor measurement. For the weighing, an electronic balance (AW220)produced by Shimadzu Co., Ltd. is used. In the case of a coated sheet,about 0.03 to 0.04 g of a net weight of a coat layer obtained bysubtracting a weight of Teflon (registered trademark) (substrate) havingthe same area from a total weight of the coated sheet is used as asample weight. Next, under the following measurement conditions, a BETspecific surface area is measured. In an adsorption side of adsorptionand desorption isotherms, a BET plot is produced from a relativepressure (P/P0) range of about 0.00 to 0.45, and a BET specific surfacearea is calculated from the slope and the intercept.

[Chem. 1] <Measurement Conditions>

-   Measurement Apparatus: BELSORP 36, High—Precise Automatic Gas    Adsorption Apparatus produced by BEL Japan, Inc.-   Adsorption Gas: N₂-   Dead Volume Measurement Gas: He-   Adsorption Temperature: 77 K (Liquid Nitrogen Temperature)-   Measurement Preparation: Vacuum Dried at 90° C. for several hours    (After He Purging, Set on Measurement Stage)-   Measurement Mode: Adsorption Process and Desorption Process in    Isotherm-   Measurement Relative Pressure P/P₀: about 0 to 0.99-   Equilibrium Setting Time: 180 sec for 1 relative pressure

In addition, the configuration (b) can sufficiently reduce an edgequantity of carbon (graphene) which exists in a graphite structure andserves as a starting point of electrochemical corrosion. Therefore, whensuch a carbon powder is used in a catalyst, durability can be improved,and decrease in catalytic activity in the support of a catalyst metalthereon can effectively suppressed and prevented. In view of furtherimprovement of the durability, the R′ value (D′/G intensity ratio) ofthe carbon powder is preferably 0 to 0.6, and more preferably 0 to 0.51.

In addition, the carbon powder according to the present inventionpreferably has a configuration (c) that a ratio R (D/G intensity ratio)of D intensity to G intensity is 1.7 or more. Since such a carbon powder(support) has a low graphitization degree, electrical double layercapacitance per surface area of the carbon powder increases, andcatalytic activity can be more effectively improved. In view of furtherimprovement of electrical double layer capacitance (catalytic activity),the R value (D/G intensity ratio) of the carbon powder is preferablymore than 1.75 and 2.5 or less, and more preferably 1.8 to 2.4.

In this description, the R′ value can be determined by obtaining a Ramanspectrum of a carbon material with a Micro-Raman spectroscope, andcalculating a relative intensity ratio between peak intensity in thevicinity of 1620 cm⁻¹ called a D′-band (D′ intensity) and peak intensityin the vicinity of 1580 cm⁻¹ called a G-band (G intensity), that is, apeak area ratio (D′ intensity/G intensity). Similarly, the R value canbe determined by obtaining a Raman spectrum of a carbon material with aMicro-Raman spectroscope, and calculating a relative intensity ratiobetween peak intensity in the vicinity of 1360 cm⁻¹ called a D-band (Dintensity) and peak intensity in the vicinity of 1580 cm⁻¹ called aG-band (G intensity), that is, a peak area ratio (D intensity/Gintensity). The peak area is obtained by Raman spectroscopic measurementto be described below.

(Raman Spectroscopic Measurement Method)

A Raman spectrum is obtained using micro laser Raman SENTERRA(manufactured by Bruker Optics) as a measurement apparatus at a roomtemperature (25° C.) for an exposure time of 30 seconds by four times asa cumulated number under the following conditions. Peaks of G-band,D′-band, and D-band can be determined by peak fitting with Gaussdistribution.

[Chemical Formula 2]

<Measurement Conditions>

Excitation wavelength: Nd: SHG of YAG, 532 nm

Laser output: 3 mW

Spot size: 1 μm or less

Detector: CCD

A size of the carbon powder is not particularly limited. From aviewpoint of easy supporting, utilization rate of a catalyst, andcontrollability of a thickness of an electrode catalyst layer within anadequate range, an average particle size (diameter) of the carbon powderis preferably in the approximate range of 5 to 2000 nm, more preferably10 to 200 nm, and particularly preferably 20 to 100 nm. As a value ofthe “average particle size of the carbon powder”, unless otherwisenoted, a value calculated as an average value of particle diameters ofparticles observed within several or several tens of fields by usingobservation means such as a scanning electron microscope (SEM) or atransmission electron microscope (TEM) is employed. In addition, the“particle size (diameter)” denotes a maximum distance among distancesbetween arbitrary two points on an outline of a particle.

As long as the carbon powder satisfies the configurations (a) and (b)described above, and particularly preferably satisfies theconfigurations (a), (b), and (c), the structure of the carbon powder isnot particularly limited. Preferably, the carbon powder furthersatisfies the following configurations (i) and (ii):

(i) the carbon powder contains pores (primary pores) having a radius ofless than 1 nm and pores (primary pores) having a radius of 1 nm ormore; and

(ii) a pore volume of pores having a radius of less than 1 nm is 0.3cc/g-support or more.

Alternatively, it is preferred that the carbon powder further satisfiesthe following configurations (i) and (iv):

(i) the carbon powder contains pores having a radius of less than 1 nmand pores having a radius of 1 nm or more; and

(iv) a mode radius of pore distribution of pores having a radius of lessthan 1 nm is 0.3 nm or more and less than 1 nm.

In this description, a pore having a radius of less than 1 nm is alsoreferred to as “micropore”. Also, in this description, a pore having aradius of 1 nm or more is referred to as “mesopore”.

By the configuration (ii) described above, a sufficient pore volume ofmicropores which a little or no electrolyte or catalyst metal can entercan be attained. By the configuration (iv) described above, a modediameter of micropores can be set to be large. For this reason, a gastransport path can be sufficiently secured in a catalyst having a thecatalyst metal supported on such a carbon powder. Accordingly, a gassuch as oxygen can be efficiently transported to a catalyst metalexisting in the mesopores. That is, gas transport resistance can bereduced. By the configuration, a gas (for example, oxygen) passesthrough the micropores (to improve gas transport ability), toefficiently contact with the catalyst.

Alternatively, it is preferred that the carbon powder further satisfiesthe above configurations (i), (ii), and (iv).

In the above configuration (ii), in view of effect of improving gastransportability, the pore volume of micropores is more preferably inthe range of 0.3 to 2 cc/g-support, further more preferably in the rangeof 0.4 to 1.5 cc/g-support, and particularly preferably in the range of0.4 to 1.0 cc/g-support. In the above configuration (iv), the moderadius of pore distribution of micropores is more preferably in therange of 0.4 to 1 nm, and particularly preferably in the range of 0.5 to0.8 nm. When the pore volume and/or mode diameter of micropores iswithin the range as described above, sufficient micropores to transporta gas can be secured, and gas transport resistance can be small.Therefore, since a sufficient amount of gas can be transported to asurface of catalyst metal existing in a mesopore(s) through amicropore(s) (path), a catalyst using the carbon powder according to thepresent invention can exhibit a higher catalytic activity, and namely,catalyst reaction can be more efficiently facilitated. In addition, anelectrolyte (ionomer) or liquid (for example, water) cannot enter amicropore, and only a gas can selectively pass through the micropore(s)(gas transport resistance can be reduced). In this description, a porevolume of pores having a radius of less than 1 nm is also simplyreferred as a “pore volume of micropores”. Similarly, in thisdescription, a mode radius of a pore distribution of micropores is alsosimply referred to as a “mode diameter of micropores”.

In addition, a pore volume of pores (mesopores) having a radius of 1 nmor more of the carbon powder is not particularly limited, but it ispreferably 0.4 cc/g-support or more, more preferably in the range of 0.4to 3 cc/g-support, further more preferably in the range of 0.4 to 1. 5cc/g-support, and particularly preferably in the range of 0.5 to 1.2cc/g-support. When the pore volume is within the range as describedabove, a larger number of catalyst metals can be placed (supported) inthe mesopores of the carbon powder, an electrolyte and a catalyst metalin a catalyst layer can be physically separated from each other (contactbetween catalyst metals and an electrolyte can be more effectivelysuppressed and prevented). Therefore, a catalyst using such a carbonpowder can more effectively use activity of the catalyst metal. Also,due to existence of a large number of mesopores, the function andeffects by the present invention can be further remarkably exhibited, sothat catalyst reaction can be more effectively facilitated. In addition,a micropore(s) functions as a gas transport path, and thus, athree-phase boundary with water is more remarkably formed, so thatcatalytic activity can be more improved. In this description, a porevolume of pores having a radius of 1 nm or more is simply referred to asa “pore volume of the mesopores”.

A mode radius (mode diameter) of a pore distribution of pores(mesopores) having a radius of 1 nm or more of the carbon powder is notparticularly limited, but it is preferably in the range of 1 to 5 nm,more preferably in the range of 1 to 4 nm, and particularly preferablyin the range of 1 to 3 nm. When the mode diameter of pore distributionof mesopores is within the range as described above, since a sufficientamount of catalyst metals can be placed (supported) in the mesopores ofthe carbon powder, an electrolyte and a catalyst metal in a catalystlayer can be physically separated from each other (contact betweencatalyst metals and an electrolyte can be more effectively suppressedand prevented). Therefore, a catalyst using such a carbon powder canmore effectively use activity of the catalyst metal. Also, due toexistence of a large volume of mesopores, the function and effects bythe present invention can be further remarkably exhibited, so thatcatalyst reaction can be more effectively facilitated. In addition, amicropore(s) functions as a gas transport path, and thus, a three-phaseboundary with water is more remarkably formed, so that catalyticactivity can be more improved. In this description, a mode radius of apore distribution of mesopores is also simply referred to as a “modediameter of the mesopores”.

The “pore radius (nm) of micropores” denotes a radius of pores measuredby a nitrogen adsorption method (MP method). In addition, the “moderadius (nm) of a pore distribution of micropores” denotes a pore radiusat a point taking a peak value (maximum frequency) in a differentialpore distribution curve obtained by a nitrogen adsorption method (MPmethod). Herein, a lower limit of the pore radius of micropores is alower limit value which can be measured by the nitrogen adsorptionmethod, that is, 0.42 nm or more. Similarly, the “pore radius (nm) ofmesopores” denotes a radius of pores measured by a nitrogen adsorptionmethod (DH method). In addition, the “mode radius (nm) of a poredistribution of mesopores” denotes a pore radius at a point taking apeak value (maximum frequency) in a differential pore distribution curveobtained by a nitrogen adsorption method (DH method). Herein, an upperlimit of the pore radius of mesopores is not particularly limited, butit is 5 nm or less.

The “pore volume of micropores” denotes a total volume of microporeshaving a radius of less than 1 nm existing in a carbon powder, and isexpressed by volume per 1 g of support (cc/g support). The “pore volume(cc/g support) of micropores” is calculated as an area (integrationvalue) under a differential pore distribution curve obtained accordingto a nitrogen adsorption method (MP method). Similarly, the “pore volumeof mesopores” denotes a total volume of mesopores having a radius of 1nm or more existing in a carbon powder, and is expressed by volume per 1g of support (cc/g support). The “pore volume (cc/g support) ofmesopores” is calculated as an area (integration value) under adifferential pore distribution curve obtained according to a nitrogenadsorption method (DH method).

The “differential pore distribution” is a distribution curve obtained byplotting a pore diameter in the horizontal axis and a pore volumecorresponding to the pore diameter in a carbon powder in the verticalaxis. Namely, when a pore volume of a carbon powder obtained by anitrogen adsorption method (MP method in case of the micropores; and DHmethod in case of the mesopores) is denoted by V and a pore diameter isdenoted by D, a value (dV/d(log D)) is obtained by dividing thedifferential pore volume dV by a differential logarithm d(log D) of thepore diameter. Next, a differential pore distribution curve is obtainedby plotting the dV/d(log D) for an average pore diameter in eachsection. A differential pore volume dV denotes an increment of porevolume between measurement points.

A method for measuring a radius and a pore volume of micropores by anitrogen adsorption method (MP method) is not particularly limited. Forexample, methods disclosed in well-down literatures such as “Science ofAdsorption” (second edition written by Kondo Seiichi, Ishikawa Tatsuo,and Abe Ikuo, Maruzen Co., Ltd.), “Fuel Cell Analysis Method” (compiledby Takasu Yoshio, Yoshitake Yu, and Ishihara Tatsumi of KAGAKU DOJIN),and an article written by R. Sh. Mikhail, S. Brunauer, and E. E. Bodorin J. Colloid Interface Sci., 26, 45 (1968) may be employed. In thisdescription, the radius and pore volume of micropores by a nitrogenadsorption method (MP method) are a value measured by the methoddisclosed in the article written by R. Sh. Mikhail, S. Brunauer, and E.E. Bodor in J. Colloid Interface Sci., 26, 45 (1968).

A method for measuring a radius and a pore volume of mesopores by anitrogen adsorption method (DH method) is not particularly limited. Forexample, methods disclosed in well-known literatures such as “Science ofAdsorption” (second edition written by Kondo Seiichi, Ishikawa Tatsuo,and Abe Ikuo, Maruzen Co., Ltd.), “Fuel Cell Analysis Method” (compiledby Takasu Yoshio, Yoshitake Yu, and Ishihara Tatsumi of KAGAKU DOJIN),and an article by D. Dollion and G. R. Heal in J. Appl. Chem. 14, 109(1964) maybe employed. In this description, the radius and pore volumeof mesopores by a nitrogen adsorption method (DH method) are a valuemeasured by the method disclosed in the article written by D. Dollionand G. R. Heal in J. Appl. Chem. 14, 109 (1964).

A method of manufacturing a carbon powder having a specific poredistribution as described above is not particularly limited.Specifically, a heat treatment method of a carbon material is preferablyused. Alternatively, a method which comprises producing a carbonmaterial having micropores and mesopores and having a pore volume ofmicropores of 0.3 cc/g-support or more by a method disclosed in, forexample, JP 2010-208887 A (corresponding to US 2011/0318254 A1), WO2009/75264 A (corresponding to US 2011/0058308 A1), or the like, andsubjecting the carbon material to heat treatment method; and a methodwhich comprises producing a carbon material having micropores andmesopores and having mode radius of pore distribution of micropores of0.3 nm or more and less than 1 nm by a method disclosed in, for example,JP 2010-208887 A, WO 2009/75264 A, or the like, and subjecting thecarbon material to heat treatment method can be preferably used.

The heat treatment conditions of carbon material is not particularlylimited as long as the above configurations (a) and (b) or the aboveconfigurations (a), (b), and (c) can be achieved. Specifically, atemperature in the heat treatment is preferably lower than 1800° C.,more preferably over 1300° C. up to 1780° C., further more preferably inthe range of 1400 to 1750° C., and particularly preferably in the rangeof 1500 to 1700° C. A temperature rising rate in the heat treatment ispreferably in the range of 100 to 1000° C./h, and particularlypreferably in the range of 300 to 800° C./h. A time in the heattreatment (holding time at a predetermined heat treatment temperature)is preferably in the range of 1 to 10 minutes, and particularlypreferably in the range of 2 to 8 minutes. The heat treatment can becarried out under an air atmosphere or an inert gas atmosphere such asargon gas or nitrogen gas. Under such conditions, the carbon powderwhich satisfies the above configurations (a) and (b) or the aboveconfigurations (a), (b), and (c) can be conveniently obtained. When theheat treatment conditions are below the lower limits (the heat treatmentconditions are too gentle), there would be a possibility that the edgequantity of carbon (graphene) cannot be sufficiently reduced.Conversely, when the heat treatment conditions are over the upper limits(the heat treatment conditions are too strict), the graphitization wouldtoo proceed, to unduly decrease a BET specific surface area of carbon(graphene).

A material of the carbon material is not particularly limited as long amain component is carbon, but is preferable to easily form a carbonpowder satisfying the BET specific surface area and the R′ valuedescribed above or the BET specific surface area, the R′ value, and theR value described above. In addition, a material which has enoughspecific surface area and enough electron conductivity to form pores(primary pores) having a pore volume or a mode diameter and to support acatalyst component inside a mesopore in a dispersed state is morepreferably used. In the latter case, the carbon material particularlypreferably satisfies the above configurations (i) and (ii) and/or (iv).Specifically, carbon particles made of carbon black (Ketjen Black, oilfurnace black, channel black, lamp black, thermal black, acetyleneblack, or the like), activated charcoal, or the like may be exemplified.The expression “main component is carbon” denotes that the carbonmaterial contains a carbon atom(s) as a main component, and includesboth of the configurations that the carbon material consists only ofcarbon atoms and that the carbon material substantially consists ofcarbon atoms. An element(s) other than carbon atom may be contained. Theexpression “substantially consists of carbon atoms” denotes thatimpurities of about 2 to 3 wt % or less can be contaminated.

A BET specific surface area of the carbon material is not particularlylimited, but is substantially similar to the BET specific surface areaof the carbon powder. A BET specific surface area of the carbon materialis 900 m²/g or more, preferably in the range of 1000 to 3000 m²/g, morepreferably in the range of 1100 to 1800 m²/g, and particularlypreferably in the range of 1200 to 1800 m²/g. With such a specificsurface area as described above, sufficient gas transportability (lowergas transport resistance) and performance (supporting a sufficientamount of catalyst metals) can be achieved.

An average particle size (average secondary particle diameter) of thecarbon material is not particularly limited, but is preferably in therange of 20 to 100 nm. From a viewpoint of easy supporting, utilizationrate of catalyst, and the like, an average particle size (averageprimary particle diameter) of the carbon material may be in the range of1 to 10 nm, and preferably in the range of 2 to 5 nm. When the averageparticle size is within such a range, even when the above-described porestructure is formed in the support, mechanical strength can bemaintained, and a catalyst layer can be controlled within an appropriaterange. As a value of the “average particle size of the carbon material”,unless otherwise specified, a value calculated as an average value ofparticle diameters of particles observed within several or several tensof fields by using observation means such as a scanning electronmicroscope (SEM) or a transmission electron microscope (TEM) isemployed. In addition, the “particle size (diameter) ” denotes a maximumdistance among distances between arbitrary two points on an outline of aparticle.

A catalyst metal which can be used in the present invention performscatalysis of electrochemical reaction. As a catalyst metal used for ananode catalyst layer, a well-known catalyst can be used in a similarmanner without particular limitation if the catalyst has catalyticeffects on oxidation reaction of hydrogen. In addition, as a catalystmetal used for a cathode catalyst layer, a well-known catalyst can beused in a similar manner without particular limitation if the catalysthas catalytic effects on reduction reaction of oxygen. Specifically, thecatalyst metal can be selected among metals such as platinum, ruthenium,iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper,silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum,gallium, and aluminum, and alloys thereof.

Among them, in view of improved catalytic activity, poison resistance tocarbon monoxide or the like, heat resistance, or the like, a catalystmetal containing at least platinum is preferably used. Namely, thecatalyst metal preferably is platinum or contains platinum and a metalcomponent(s) other than the platinum, more preferably is platinum or aplatinum-containing alloy. Such a catalyst metal can exhibit highactivity. Particularly, when the catalyst metal is platinum, sinceplatinum having a small particle size can be dispersed on a surface ofcarbon powder (support), platinum surface area per unit weight can bemaintained even with a reduced amount of platinum. In addition, when thecatalyst metal includes platinum and a metal component(s) other thanplatinum, since an amount of expensive platinum to be used can bereduced, which is preferred in view of costs. Although a composition ofan alloy depends on a kind of the metal constituting the alloy, acontent of platinum may be in the range of 30 to 90 atom %, and acontent of a metal constituting the alloy together with platinum may bein the range of 10 to 70 atom %. In general, an alloy is obtained bymixing a metal element with at least one metal element or non-metalelement, and is a general term for substances having metallicproperties. The structure of the alloy includes an eutectic alloy whichis a mixture where component elements form separate crystals, an alloywhere component elements are completely fused to form a solid solution,an alloy where component elements form a intermetallic compound or acompound between a metal and a non-metal, and the like, and any onethereof may be employed in the present application. A catalyst metalused in an anode catalyst layer and a catalyst metal used in a cathodecatalyst layer can be appropriately selected from the aforementionedalloys. In this description, unless otherwise noted, the description ofthe catalyst metal for the anode catalyst layer and the catalyst metalfor the cathode catalyst layer have the same definition. However, thecatalyst metal for the anode catalyst layer and the catalyst metal forthe cathode catalyst layer are not necessarily the same, and thecatalyst metals can be appropriately selected so that the desiredfunctions described above can be attained.

A shape and size of the catalyst metal (catalyst component) are notparticularly limited, but the shapes and sizes of well-known catalystcomponents may be employed. As the shape, for example, a granular shape,a squamous shape, a laminar shape, or the like may be used, but thegranular shape is preferred. In this case, an average particle size(diameter) of the catalyst metal (catalyst metal particle) is notparticularly limited, but is 3 nm or more, preferably more than 3 nm and30 nm or less, and particularly preferably more than 3 nm and 10 nm orless. When the average particle size of the catalyst metal is 3 nm ormore, the catalyst metal is relatively strongly supported on the carbonpowder (for example, in a mesopore of the carbon powder) and can be moreeffectively suppressed and prevented from contacting with an electrolytein a catalyst layer. Further, when the carbon powder has micropores, themicropores are not blocked by the catalyst metals but remain, and thus,a gas transport path can be more efficiently secured, so that gastransport resistance can be further reduced. In addition, elution due toa change in voltage can be prevented, and temporal degradation inperformance can be also suppressed. Therefore, catalytic activity can befurther improved, namely, catalyst reaction can be more efficientlyfacilitated. On the other hand, if the average particle diameter of thecatalyst metal particles is 30 nm or less, the catalyst metals can besupported on the carbon powder (for example, inside the mesopores of thecarbon powder) in a simple manner, so that a coating ratio of catalystmetals with an electrolyte can be reduced. In the present invention, the“average particle size of catalyst metal particles” can be measured asan average value of a crystallite diameter obtained from a half-valuewidth of a diffraction peak of a catalyst metal component in X-raydiffraction or as an average value of a particle size of catalyst metalparticle examined from a transmission electron microscope (TEM).

In this embodiment, a catalyst metal content per unit catalyst-coatedarea (mg/cm²) is not particularly limited so long as a sufficientdispersibility of catalyst on a support and power generation performancecan be obtained. For example, the catalyst metal content is in the rangeof 0.01 to 1 mg/cm². However, in the case where the catalyst containsplatinum or a platinum-containing alloy, a platinum content per unitcatalyst-coated area is preferably 0.5 mg/cm² or less. The usage ofexpensive noble metal catalyst represented by platinum (Pt) or aplatinum alloy induces an increased cost of a fuel cell. Therefore, itis preferable to reduce the cost by decreasing an amount of theexpensive platinum (platinum content) to the above-described range. Alower limit is not particularly limited so long as power generationperformance can be attained, and for example, the lower limit value is0.01 mg/cm² or more. The platinum content is more preferably in therange of 0.02 to 0.4 mg/cm². In this embodiment, since the activity percatalyst weight can be improved by controlling the pore structure of thesupport, the amount of an expensive catalyst can be reduced.

In this description, an inductively coupled plasma emission spectroscopy(ICP) is used for measurement (determination) of a “content of catalyst(platinum) per unit catalyst-coated area (mg/cm2)”. A method ofobtaining a desired “content of catalyst (platinum) per unitcatalyst-coated area (mg/cm2)” can be easily performed by the personskilled in the art, and the content can be adjusted by controlling aslurry composition (catalyst concentration) and a coated amount.

A supported amount (in some cases, referred to as a support ratio) of acatalyst on a support is preferably in the range of 10 to 80 wt %, morepreferably in the range of 20 to 70 wt %, with respect to a total amountof the catalyst support (that is, the support and the catalyst). Thesupported amount within the aforementioned range is preferable in termsof sufficient dispersibility of a catalyst component on a support,improved power generation performance, economical merit, and catalyticactivity per unit weight.

As long as the carbon powder satisfies the configurations (a) and (b)described above, and particularly preferably satisfies theconfigurations (a), (b), and (c), a structure of the catalyst is notparticularly limited. However, as described above, particularlypreferably the carbon powder further satisfies the followingconfigurations (i) and (ii) and/or (iv). In this case, the catalyst(also referred to as an “electrode catalyst” in this description)includes the carbon powder (catalyst support) of the present inventionand a catalyst metal to be supported on the carbon powder, andpreferably satisfies the following configurations (i) to (iii):

(i) the catalyst includes pores (primary pores) having a radius of lessthan 1 nm and pores (primary pores) having a radius of 1 nm or more;

(ii) a pore volume of the pores having the radius of less than 1 nm is0.3 cc/g-support or more; and

(iii) at least a part of the catalyst metals are supported inside thepores having a radius of 1 nm or more.

Similarly, the catalyst (also referred to as an “electrode catalyst” inthis description) includes the carbon powder (catalyst support) of thepresent invention and a catalyst metal to be supported on the carbonpowder, and preferably satisfies the following configurations (i), (iv),and (iii):

(i) the catalyst includes pores having a radius of less than 1 nm andpores having a radius of 1 nm or more;

(iv) a mode radius of pore distribution of pores having a radius of lessthan 1 nm is 0.3 nm or more and less than 1 nm; and

(iii) at least a part of the catalyst metals are supported inside a porehaving a radius of 1 nm or more.

The present inventors have found that, even in the case where a catalystis not in contact with an electrolyte, the catalyst forms a three-phaseboundary with and water, so that the catalyst can be effectively used.Thus, the catalytic activity can be improved by taking the configuration(iii) where a catalyst metal(s) is supported inside a mesopore(s) whichan electrolyte cannot enter.

On the other hand, in the case where a catalyst metal(s) is supportedinside a mesopore(s) which an electrolyte cannot enter, a transportingdistance of a gas such as oxygen is increased, and thus, gastransportability is deteriorated. Therefore, a sufficient catalyticactivity cannot be exhibited, and catalyst performance is deterioratedunder high load conditions. On the contrary, by securing a sufficientpore volume of micropores in the configuration (ii) or by setting a moderadius of micropore to be large in the configuration (iv), sufficientpath for transporting a gas can be secured. Thus a gas such as oxygencan be efficiently transported to a catalyst in a mesopore, and namely,gas transport resistance can be reduced. Due to this configuration, agas (for example, oxygen) can pass through the micropores (gastransportability is improved), and the gas may be allowed to beefficiently in contact with the catalyst.

Therefore, when the catalyst satisfies the above configurations (i) to(iii) or the above configurations (i), (iv) and (iii), since themicropores exist with a large volume, a reaction gas can be transportedthrough the micropores (paths) to a surface of catalyst metal existingin the mesopore, gas transport resistance can decrease. Therefore, thecatalyst can exhibit a high catalytic activity, and namely, the catalystreaction can be facilitated. For this reason, the membrane electrodeassembly and fuel cell comprising the catalyst layer using the catalystaccording to the present invention have an excellent power generationperformance.

Hereinafter, an embodiment satisfying the above configurations (i) to(iii) or the above configurations (i), (iv), and (iii) will be describedin detail. However, the following embodiment is a preferred form of thepresent invention, and the present invention is not limited to thefollowing embodiment.

FIG. 2 is an explanatory cross-sectional diagram schematicallyillustrating a shape and a structure of the catalyst satisfying theabove configurations (i) to (iii) or the above configurations (i), (iv)and (iii). As illustrated in FIG. 2, a catalyst 20 is composed ofcatalyst metal 22 and catalyst support (carbon powder of the presentinvention) 23. The catalyst 20 includes pores (micropores) 25 having aradius of less than 1 nm and pores (mesopores) 24 having a radius of 1nm or more. The catalyst metal 22 is supported inside the mesopore 24.In addition, at least a portion of the catalyst merals 22 may besupported inside the mesopores 24, and at least a portion thereof may besupported on the surface of the catalyst support 23. However, in view ofpreventing the catalyst metal from being contacted with an electrolytein a catalyst layer, it is preferable that substantially all thecatalyst merals 22 be supported inside the mesopores 24. As used herein,the expression “substantially all the catalyst metals” is notparticularly limited if an amount which can improve a sufficientcatalytic activity can be attained. The amount of “substantially all thecatalyst metals” is preferably 50 wt % or more (upper limit: 100 wt %),more preferably 80 wt % or more (upper limit: 100 wt %), with respect toall the catalyst metals.

In this description, the state “the catalyst metals are supported insidethe mesopores” can be confirmed by a decrease in volume of mesoporesbefore and after the supporting of catalyst metals on a catalystsupport. Specifically, a catalyst support (hereinafter, also simplyreferred to as a “support”) contains micropores and mesopores, and thepores have the respective certain volumes. If catalyst metals aresupported in the pore(s), the volumes of the pores are decreased.Therefore, the case where a difference between a volume of mesopores ofa catalyst (support) before the supporting of catalyst metals and avolume of mesopores of a catalyst (support) after the supporting ofcatalyst metals [=(volume before supporting)−(volume after supporting)]exceeds 0 indicates that “the catalyst metals are supported inside themesopore(s)”. Similarly, the case where a difference between a volume ofmicropores of a catalyst (support) before the supporting of catalystmetals and a volume of micropores of a catalyst (support) after thesupporting of catalyst metals [=(volume before supporting)−(volume aftersupporting)] exceeds 0 indicates that “the catalyst metals are supportedinside the micropore(s)”. Preferably, a larger number of catalyst metalsare supported in mesopores than in micropores (namely, (decreased volumeof mesopores before and after the supporting)>(decreased volume ofmicropores before and after the supporting)). By this, gas transportresistance can be reduced and a path for gas transportation can besufficiently secured. In terms of reduced gas transport resistance andsecuring of a path for gas transportation, a decreased pore volume ofmesopores before and after the supporting of the catalyst metals ispreferably 0.02 cc/g or more, more preferably in the range of 0.02 to0.21 cc/g.

A pore volume of pores (micropores) having a radius of less than 1 nm ofa catalyst (after catalyst metal(s) is supported) is 0.3 cc/g support ormore, and/or a mode radius of a pore distribution of micropores of acatalyst (after catalyst metal(s) is supported) (maximum frequencydiameter) is 0.3 nm or more and less than 1 nm. Preferably, the porevolume of micropores is 0.3 cc/g support or more, and the mode radius ofthe pore distribution of micropores is 0.3 nm or more and less than 1nm. If the pore volume of micropores and/or the mode diameter are withinsuch ranges, enough micropores for gas transportation can be secured, sothat gas transport resistance becomes small. Therefore, since asufficient amount of a gas can be transported to a surface(s) ofcatalyst metal(s) existing in the mesopores via micropores (path), thecatalyst according to the present invention can exhibit a high catalystactivity, and namely, the catalyst reaction can be facilitated. Inaddition, an electrolyte (ionomer) or liquid (for example, water) cannotenter the micropores, and only a gas can selectively pass through themicropores (gas transport resistance can be reduced). In terms ofeffects of improving gas transportability, the pore volume of microporesis more preferably in the range of 0.3 to 2 cc/g support, particularlypreferably in the range of 0.4 to 1.5 cc/g support. The mode radius ofthe pore distribution of micropores is more preferably in the range of0.4 to 1 nm, particularly in the range of 0.4 to 0.8 nm.

A pore volume of the pores (mesopores) having a radius of 1 nm or moreof a catalyst (after catalyst metal(s) is supported) is not particularlylimited, but it is preferably 0.4 cc/g support or more, more preferablyin the range of 0.4 to 3 cc/g support, particularly preferably in therange of 0.4 to 1.5 cc/g support. If the pore volume is within such arange, a larger number of catalyst metals can be placed (supported) inthe mesopores, and thus, an electrolyte and a catalyst metal(s) in thecatalyst layer are physically separated from each other (contact of acatalyst metal(s) and an electrolyte can be more effectively suppressedand prevented). Therefore, activity of the catalyst metals can be moreeffectively used. In addition, due to existence of a large number ofmesopores, the function and effects by the present invention can befurther remarkably exhibited, so that a catalyst reaction can be moreeffectively facilitated. Also, the micropores function as a gastransport path, and thus, three phase boundary with water can be moreremarkably formed, so that the catalytic activity can be more improved.

A mode radius (maximum frequent diameter) of a pore distribution ofpores (mesopores) having a radius of 1 nm or more of a catalyst (aftercatalyst metal(s) is supported) is not particularly limited, but it ispreferably in the range of 1 to 5 nm, more preferably in the range of 1to 4 nm, particularly preferably in the range of 1 to 3 nm. If the modediameter of the pore distribution of mesopores is within such a range, asufficient number of catalyst metals can be placed (supported) in themesopores, and thus, an electrolyte and a catalyst metal(s) in thecatalyst layer are physically separated from each other (contact of acatalyst metal(s) and an electrolyte can be more effectively suppressedand prevented). Therefore, activity of the catalyst metals can be moreeffectively used. In addition, due to existence of a large volume ofmesopores, the function and effects by the present invention can befurther remarkably exhibited, so that a catalyst reaction can be moreeffectively facilitated. Also, the micropores function as a gastransport path, and thus, three phase boundary with water can be moreremarkably formed, so that the catalytic activity can be more improved.

A BET specific surface area of a catalyst (after catalyst metal(s) issupported) [BET specific surface area of catalyst per 1 g of support(m²/g support)] is not particularly limited, but is 900 m²/g support ormore, more preferably in the range of 1000 to 3000 m²/g support,particularly preferably in the range of 1100 to 1800 m²/g support. Ifthe specific surface area is within the above-described range, sincesufficient mesopores and micropores can be secured, enough micropores totransport a gas (lower gas transport resistance) can be secured, and alarger number of the catalyst metals can be placed (supported) in themesopores. In addition, an electrolyte and catalyst metals in thecatalyst layer can be physically separated from each other (contactbetween catalyst metals and an electrolyte can be more effectivelysuppressed and prevented). Therefore, activity of the catalyst metalscan be more effectively used. Also, due to existence of a large numberof the micropores and mesopores, the function and effects by the presentinvention can be further remarkably exhibited, so that a catalystreaction can be more effectively facilitated. In addition, themicropores function as a gas transport path, and thus, a three-phaseboundary with water is more remarkably formed, so that the catalyticactivity can be more improved.

A method of manufacturing a catalyst (method for supporting a catalystmetal on a carbon powder) according to the present invention is notparticularly limited. Preferably, a method which comprises precipitatinga catalyst metal on a surface of catalyst support, and thereafterperforming heat treatment to increase a particle diameter of thecatalyst metal is preferred. The method increases a granular size of thecatalyst metals by the heat treatment after the precipitating.Therefore, the catalyst metals having a large particle diameter can besupported in pores (particularly, mesopores) of catalyst support.Namely, the present invention also provides a method of manufacturingthe catalyst according to the present invention, which includes (i) astep (precipitation step) of precipitating a catalyst metal on a surfaceof a catalyst support and (ii) a step (heat treatment step) of, afterthe precipitation step, performing heat treatment to increase a particlediameter of the catalyst metal. Hereinafter, the method will bedescribed, but the present invention is not limited to the followingform.

Hereinafter, a preferred embodiment of the method of manufacturing acatalyst will be described, but the present invention is not limited tothe following embodiment.

(i) Precipitation Step

In this step, a catalyst metal(s) is allowed to be precipitated on asurface(s) of a catalyst support(s). The step has been known in the artand, for example, a method of immersing the catalyst supports in aprecursor solution of the catalyst metal and, after that, performingreduction is preferably used.

Herein, a precursor of catalyst metal is not particularly limited, butit is appropriately selected according to a kind of the catalyst metalwhich is to be used. Specifically, chlorides, nitrates, sulfates,chlorides, acetates, amine compounds or the like of the catalyst metalssuch as platinum as described above may be exemplified. Morespecifically, chlorides such as platinum chloride (hexachloroplatinicacid hexahydrate), palladium chloride, rhodium chloride, rutheniumchloride, and cobalt chloride, nitrates such as palladium nitrate,rhodium nitrate, and iridium nitrate, sulfates such as palladium sulfateand rhodium sulfate, acetates such as rhodium acetate, ammine compoundssuch as dinitrodiammine platinum nitric acid and dinitrodiamminepalladium, or the like may be preferably exemplified. In addition, asolvent used for manufacturing the precursor solution of catalyst metalis not particularly limited so long as the solvent can dissolve theprecursor of catalyst metal. The solvent is appropriately selectedaccording to a kind of the precursor of catalyst metal which is to beused. Specifically, water, acids, alkalis, organic solvents, or the likemay be exemplified. A concentration of the precursor of catalyst metalin the precursor solution of the catalyst metal is not particularlylimited, but is preferably in the range of 0.1 to 50 wt %, morepreferably in the range of 0.5 to 20 wt %, in terms of metal.

As a reducing agent, hydrogen, hydrazine, sodium hydroborate, sodiumthiosulfate, citric acid, sodium citrate, L-ascorbic acid, sodiumborohydride, formaldehyde, methanol, ethanol, ethylene, carbon monoxide,or the like may be exemplified. A material which is gaseous at roomtemperature such as hydrogen can be supplied by bubbling. An amount ofthe reducing agent is not particularly limited so long as the precursorof catalyst metal can be reduced to the catalyst metal, and a well-knownamount can be applied in the same manner.

Precipitation conditions are not particularly limited so long as thecatalyst metal can be precipitated on the catalyst support. For example,a precipitation temperature is preferably a temperature around theboiling point of the solvent, more preferably in the range of roomtemperature to 100° C. A precipitation time is preferably in the rangeof 1 to 10 hours, more preferably in the range of 2 to 8 hours. Theprecipitation step may be performed while stirring and mixing ifnecessary.

By the step, the precursor of the catalyst metal is reduced to thecatalyst metal, so that the catalyst metal is precipitated (supported)on the catalyst support.

(ii) Heat Treatment Step

In this step, after the (i) precipitation step, heat treatment isperformed to increase a particle diameter of the catalyst metals.

Heat treatment conditions are not particularly limited so long as aparticle diameter of the catalyst metals increase. For example, a heattreatment temperature is preferably in the range of 300 to 1200° C.,more preferably in the range of 500 to 1150° C., particularly preferablyin the range of 700 to 1000° C. A heat treatment time is preferably inthe range of 0.02 to 3 hours, more preferably in the range of 0.1 to 2hours, particularly preferably in the range of 0.2 to 1.5 hours. Theheat treatment step may be performed in a hydrogen ambience.

By this step, the catalyst metal increases its particle diameter in thecatalyst support (particularly, in the mesopores of the catalystsupport). Therefore, the catalyst metal particles are hardly detachedoutside (from the catalyst supports). In addition, due to themicrospores existing in the vicinity of the surface of the catalystsupport from the catalyst metal, the catalyst metals having a largersize can be more effectively suppressed and prevented from beingdetached from the catalyst support even under a mechanical stress.Therefore, the catalyst can be more effectively used.

[Catalyst Layer]

As described above, the catalyst of the present invention can reduce gastransport resistance, so that the catalyst can exhibit a high catalyticactivity and in other words, catalyst reaction can be promoted.Therefore, the catalyst of the present invention can be advantageouslyused for an electrode catalyst layer for fuel cell. Namely, the presentinvention provides an electrode catalyst layer for fuel cell includingthe electrode catalyst according to the present invention and anelectrolyte. The electrode catalyst layer for fuel cell can manifesthigh performance and durability.

The electrode catalyst layer for fuel cell of the present invention canbe applied in the same manner as in a conventional manner or byappropriately modifying a conventional manner except for using thecarbon powder of the present invention as a support. Accordingly, apreferred embodiment of the catalyst layer will be described below, butthe present invention is not limited to the following embodiment.

FIG. 3 is a schematic diagram illustrating an example of the relationbetween a catalyst and an electrolyte in a catalyst layer in the casewhere the carbon powder is as illustrated in FIG. 2.

As illustrated in FIG. 3, in the catalyst layer, although the catalystis coated with the electrolyte 26, the electrolyte 26 does not enter themesopores 24 (and the micropores 25) of the catalyst (support 23).Therefore, although the catalyst metal 22 on the surface of the support23 is in contact with the electrolyte 26, the catalyst metal 22supported in the mesopore 24 is not in contact with the electrolyte 26.The catalyst metal in the mesopore forms three-phase boundary with anoxygen gas and water in a state that the catalyst metal is not incontact with the electrolyte, so that a reaction active area of thecatalyst metals can be secured.

Although the catalyst according to the present invention may existeither in a cathode catalyst layer or an anode catalyst layer, thecatalyst is preferably used in a cathode catalyst layer. As describedabove, although the catalyst according to the present invention is notin contact with the electrolyte, the catalyst can be effectively used byforming three-phase boundary of the catalyst and water. This is becausewater is formed in the cathode catalyst layer.

An electrolyte is not particularly limited, but it is preferably anion-conductive polymer electrolyte. Since the polymer electrolyte servesto transfer protons generated in the vicinity of the catalyst activematerial on a fuel electrode side, the polymer electrolyte is alsoreferred to as a proton conductive polymer.

The polymer electrolyte is not particularly limited, but well-knownknowledge in the art can be appropriately referred to. The polymerelectrolytes are mainly classified into fluorine-based polymerelectrolytes and hydrocarbon-based polymer electrolytes depending on atype of an ion-exchange resin as a constituent material.

As an ion-exchange resin constituting the fluorine-based polymerelectrolyte, for example, perfluorocarbon sulfonic acid based polymerssuch as Nafion (registered trademark, produced by DuPont), Aciplex(registered trademark, produced by Asahi Kasei Co., Ltd.), and Flemion(registered trademark, produced by Asahi Glass Co., Ltd.),perfluorocarbon phosphoric acid based polymers, trifluorostyrenesulfonic acid based polymers, ethylene tetrafluoroethylene-g-styrenesulfonic acid based polymers, ethylene-tetrafluoroethylene copolymers,polyvinylidene fluoride-perfluorocarbon sulfonic acid based polymers,and the like may be exemplified. In terms excellent heat resistance,chemical stability, durability, and mechanical strength, thefluorine-based polymer electrolyte is preferably used, and afluorine-based polymer electrolyte formed of a perfluorocarbon sulfonicacid based polymer is particularly preferably used.

As a hydrocarbon-based electrolyte, sulfonated polyether sulfones(S-PES), sulfonated polyaryl ether ketones, sulfonated polybenzimidazolealkyls, phosphonated polybenzimidazole alkyls, sulfonated polystyrenes,sulfonated polyether ether ketones (S-PEEK), sulfonated polyphenylenes(S-PPP), and the like may be exemplified. In terms of manufacturingadvantages such as inexpensive raw materials, simple manufacturingprocesses, and high selectivity of materials, a hydrocarbon-basedpolymer electrolyte is preferably used. These ion-exchange resins may besingly used, or two or more resins may be used together. In addition,the material is not limited to the above-described material, but anothermaterial may be used.

With respect to the polymer electrolyte which serves to transferprotons, proton conductivity is important. In the case where EW of apolymer electrolyte is too large, ion conductivity with in the entirecatalyst layer would be decreased. Therefore, the catalyst layeraccording to the embodiment preferably includes a polymer electrolytehaving a small EW. Specifically, catalyst layer according to theembodiment preferably includes a polymer electrolyte having an EW of1500 g/eq. or less, more preferably includes a polymer electrolytehaving an EW of 1200 g/eq. or less, and particularly preferably includesa polymer electrolyte having an EW of 1000 g/eq. or less.

On the other hand, in the case where the EW is too small, sincehydrophilicity is too high, water is hard to smoothly move. Due to sucha point of view, the EW of polymer electrolyte is preferably 600 ormore. The EW (Equivalent Weight) represents an equivalent weight of anexchange group having proton conductivity. The equivalent weight is adry weight of an ion exchange membrane per 1 eq. of ion exchange group,and is represented in units of “g/eq.”.

It is preferable that the catalyst layer includes two types or more ofpolymer electrolytes having different EWs in a power generation surface,and in this case, among the polymer electrolytes, the polymerelectrolyte having the lowest EW is used in an area where relativehumidity of a gas in a passage is 90% or less. By employing suchmaterial arrangement, resistance is decreased irrespective of a currentdensity area, so that cell performance can be improved. The EW ofpolymer electrolyte used in the area where relative humidity of the gasin a passage is 90% or less, that is, EW of polymer electrolyte havingthe lowest EW is preferably 900 g/eq. or less. By this, theabove-described effects can be further more certainly and moreremarkably attained.

The polymer electrolyte having the lowest EW is preferably used in anarea of which temperature is higher than an average temperature of inletand outlet for cooling water. By this, resistance is decreasedirrespective of a current density area, so that cell performance can befurther improved.

In terms decreased resistance value of a fuel cell system, the polymerelectrolyte having the lowest EW is preferably provided in an areawithin the range of 3/5 or less of the passage length from a gas supplyinlet of at least one of a fuel gas and an oxidant gas.

The catalyst layer according to the embodiment may include, between thecatalyst and the polymer electrolyte, a liquid proton conductingmaterial capable of connecting the catalyst and the polymer electrolytein a proton conductible state. By introducing the liquid protonconducting material, a proton transport path through the liquid protonconducting material is provided between the catalyst and the polymerelectrolyte, so that protons necessary for the power generation can beefficiently transported on the surface of the catalyst. By this,availability of the catalyst is improved, and thus an amount of usedcatalyst can be reduced while maintaining power generation performance.The liquid proton conducting material may be interposed between thecatalyst and the polymer electrolyte. The liquid proton conductingmaterial may be disposed in pores (secondary pores) between poroussupports in a catalyst layer or may be disposed in pores (micropores ormesopores: primary pores) in porous supports.

The liquid proton conducting material is not particularly limited if thematerial has ion conductivity and has a function of forming a protontransport path between the catalyst and the polymer electrolyte.Specifically, water, aprotic ionic liquid, an aqueous solution ofperchloric acid, an aqueous solution of nitric acid, an aqueous solutionof formic acid, an aqueous solution of acetic acid, and the like may beexemplified.

In the case of using water as the liquid proton conducting material, thewater can be introduced as the liquid proton conducting material intothe catalyst layer by wetting the catalyst layer with a small amount ofliquid water or a humidified gas before the start of power generation.In addition, water generated through electrochemical reaction during theoperation of a fuel cell may be used as the liquid proton conductingmaterial. Therefore, in a state where a fuel cell starts to be operated,the liquid proton conducting material is not necessarily retained. Forexample, a surface distance between the catalyst and the electrolyte ispreferably set to be a diameter of an oxygen ion constituting a watermolecule, that is, 0.28 nm or more. By maintaining such a distance,water (liquid proton conducting material) can be interposed between thecatalyst and the polymer electrolyte (in the liquid conducting materialretaining portion) while maintaining the non-contact state between thecatalyst and the polymer electrolyte, so that a proton transport pathcan be secured by water therebetween.

In the case of using a material such as an ionic liquid other than wateras the liquid proton conducting material, the ionic liquid, the polymerelectrolyte, and the catalyst are preferably allowed to be dispersed ina solution in the preparation of a catalyst ink. However, the ionicliquid may be added at the time of coating a catalyst layer substratewith a catalyst.

In the catalyst according to the present invention, a total area of thecatalyst which is in contact with the polymer electrolyte is set to besmaller than a total area of the catalyst exposed to the liquidconducting material retaining portion.

Comparison of these areas can be performed, for example, by obtaining amagnitude relationship between capacitance of an electrical double layerformed in a catalyst-polymer electrolyte interface and capacitance of anelectrical double layer formed in a catalyst-liquid proton conductingmaterial interface in a state where the liquid conducting materialretaining portion is filled with the liquid proton conducting material.Namely, since capacitance of an electrical double layer is proportionalto an area of an electrochemically effective interface, if thecapacitance of the electrical double layer formed in thecatalyst-electrolyte interface is smaller than the capacitance of theelectrical double layer formed in the catalyst-liquid proton conductingmaterial interface, a contact area of the catalyst with the electrolyteis smaller than an area thereof exposed to the liquid conductingmaterial retaining portion.

Herein, a measuring method for capacitance of an electrical double layerformed in a catalyst-electrolyte interface and capacitance of anelectrical double layer formed in a catalyst-liquid proton conductingmaterial interface, that is, a magnitude relationship between a contactarea of the catalyst with the electrolyte and a contact area of thecatalyst and the liquid proton conducting material (determination methodfor a magnitude relationship between a contact area of the catalyst andthe electrolyte and an area of the catalyst exposed to the liquidconducting material retaining portion) will be described.

Namely, in the catalyst layer according to the embodiment, the followingfour types of interfaces can contribute as capacitance of electricaldouble layer (Cdl):

(1) catalyst-polymer electrolyte (C-S)

(2) catalyst-liquid proton conducting material (C-L)

(3) porous support-polymer electrolyte (Cr-S)

(4) porous support-liquid proton conducting material (Cr-L)

As described above, since capacitance of an electrical double layer isproportional to an area of an electrochemically effective interface,CdlC-S (capacitance of an electrical double layer in a catalyst-polymerelectrolyte interface) and CdlC-L (capacitance of an electrical doublelayer in a catalyst-liquid proton conducting material interface) may beobtained. Therefore, the contribution of the four types of interfaces tocapacitance of an electrical double layer (Cdl) can be identified asfollows.

First, for example, under a high humidity condition such as 100% RH andunder a lower humidity condition such as 10% RH or less, eachcapacitance of electrical double layers is measured. As a measurementmethod for the capacitance of electrical double layer, cyclicvoltammetry, electrochemical impedance spectroscopy, or the like may beexemplified. From the comparison, the contribution of the liquid protonconducting material (in this case, “water”), that is, theabove-described contributions (2) and (4) can be identified.

In addition, the contributions to capacitance of an electrical doublelayer can be identified by deactivating a catalyst, for example, in thecase of using Pt as the catalyst, by deactivating the catalyst by supplyCO gas to an electrode to be measured to allow CO to be adsorbed on thesurface of Pt. In this state, as described above, under the highhumidity condition and under the low humidity condition, eachcapacitance of electrical double layers is measured by the same method,and from the comparison, the contributions of the catalyst, that is, theabove-described contributions (1) and (2) can be identified.

By using the above-described method, all the contributions (1) to (4)described above can be identified, the capacitance of the electricaldouble layer in the interface between the catalyst and the polymerelectrolyte and the capacitance of the electrical double layer in theinterface between the catalyst and the liquid proton conducting materialcan be obtained.

Namely, a measurement value (A) in a highly-humidified state can beregarded as capacitance of electrical double layer formed in all theinterfaces (1) to (4), and a measurement value (B) in a lowly-humidifiedstate can be regarded as capacitance of the electrical double layerformed in the interfaces (1) and (3). In addition, a measurement value(C) in a catalyst-deactivated and highly-humidified state can beregarded as capacitance of the electrical double layer formed in theinterfaces (3) and (4), and a measurement value (D) in acatalyst-deactivated and lowly-humidified state can be regarded ascapacitance of the electrical double layer formed in the interface (3).

Therefore, the difference between A and C can be regarded as thecapacitance of the electrical double layer formed in the interfaces (1)and (2), and the difference between B and D can be regarded as thecapacitance of the electrical double layer formed in the interface (1).Next, by calculating the difference between these values, i.e.,(A−C)−(B−D), the capacitance of the electrical double layer formed inthe interface (2) can be obtained. In addition, a contact area of thecatalyst with the polymer electrolyte or an exposed area thereof to theconducting material retaining portion can be obtained by, for example,TEM (transmission electron microscope) tomography besides theabove-described method.

If necessary, the catalyst layer may contain additives of a waterrepellent such as polytetrafluoroethylene, polyhexafluoropropylene, andtetrafluoroethylene-hexafluoropropylene copolymer, a dispersant such asa surfactant, a thickener such as glycerin, ethylene glycol (EG),polyvinyl alcohol (PVA), and propylene glycol (PG), a pore-formingagent, or the like.

A thickness (as a dried thickness) of the catalyst layer is preferablyin the range of 0.05 to 30 μm, more preferably in the range of 1 to 20μm, even more preferably in the range of 2 to 15 μm. The thickness canbe applied to both of the cathode catalyst layer and the anode catalystlayer. However, the thickness of the cathode catalyst layer and thethickness of the anode catalyst layer may be equal to or different fromeach other.

(Method of Manufacturing Catalyst Layer)

Hereinafter, a method for manufacturing the catalyst layer will bedescribed as an exemplary embodiment, but the scope of the presentinvention is not limited to the following embodiment. In addition, allthe conditions for the components and the materials of the catalystlayer are as described above, and thus, the description thereof isomitted.

First, a carbon powder as a support (in this description, also referredto as a “porous support” or a “conductive porous support”) is prepared.Specifically, the support may be produced as described above in themethod of producing the carbon powder.

Next, the catalyst is supported on the porous support, so that acatalyst powder is prepared. The supporting of the catalyst on theporous support can be performed by a well-known method. For example, awell-known method such as an impregnation method, a liquid phasereduction supporting method, an evaporation drying method, a colloidadsorption method, a spray pyrolysis method, or reverse micelle(micro-emulsion method) may be used.

Subsequently, a catalyst ink containing the catalyst powder, polymerelectrolyte, and a solvent is prepared. As the solvent, there is noparticular limitation. A typical solvent used for forming a catalystlayer may be similarly used. Specifically, water such as tap water, purewater, ion-exchanged water, distilled water, cyclohexanol, a loweralcohol having 1 to 4 carbons such as methanol, ethanol, n-propanol,isopropanol, n-butanol, sec-butanol, isobutanol, and tert-butanol,propylene glycol, benzene, toluene, xylene, or the like may be used.Besides, acetic acid butyl alcohol, dimethyl ether, ethylene glycol, orthe like may be used as a solvent. These solvents may be used alone ormay be used in a state of a mixture of two or more solvents.

An amount of solvent for preparing the catalyst ink is not particularlylimited so long as the electrolyte can be completely dissolved.Specifically, a concentration (a solid content) of the catalyst powderand the polymer electrolyte is preferably in the range of 1 to 50 wt %in the electrode catalyst ink, more preferably in the range of about 5to 30 wt %.

In the case of using an additive such as a water repellent, adispersant, a thickener, and a pore-forming agent, the additive may beadded to the catalyst ink. In this case, an added amount of the additiveis not particularly limited so long as it does not interfere with theabove-described effects by the present invention. For example, the addedamount of the additive is preferably in the range of 5 to 20 wt %, withrespect to the total weight of the electrode catalyst ink.

Next, a surface of a substrate is coated with the catalyst ink. A methodof coating the substrate is not particularly limited, but a well-knownmethod may be used. Specifically, a well-known method such as a spray(spray coat) method, a Gulliver printing method, a die coater method, ascreen printing method, or a doctor blade method can be used.

As the substrate coated with the catalyst ink, a solid polymerelectrolyte membrane (electrolyte layer) or a gas diffusion substrate(gas diffusion layer) may be used. In this case, after the catalystlayer is formed on a surface of a solid polymer electrolyte membrane(electrolyte layer) or a gas diffusion substrate (gas diffusion layer),the resultant laminate maybe used as it is for manufacturing a membraneelectrode assembly. Alternatively, as the substrate, a peelablesubstrate such as a polytetrafluoroethylene (PTFE) [Teflon (registeredtrademark)] sheet can be used, and after a catalyst layer is formed onthe substrate, the catalyst layer portion can be peeled off from thesubstrate, so that the catalyst layer may be obtained.

Finally, the coat layer (film) of the catalyst ink is dried under an airambience or under an inert gas ambience at a temperature ranging fromroom temperature to 150° C. for a time ranging from 1 to 60 minutes. Bythis, the catalyst layer can be formed.

(Membrane Electrode Assembly/Fuel Cell)

According to another embodiment of the present invention, provided is amembrane electrode assembly for fuel cell including the above-describedelectrode catalyst layer for fuel cell and a fuel cell including themembrane electrode assembly for fuel cell. That is, provided is amembrane electrode assembly for fuel cell which includes a solid polymerelectrolyte membrane 2, a cathode catalyst layer disposed at one side ofthe electrolyte membrane, an anode catalyst layer disposed at the otherside of the electrolyte membrane, and a pair of gas diffusion layers (4a, 4 c) interposing the electrolyte membrane 2, the anode catalyst layer3 a, and the cathode catalyst layer 3 c. In the membrane electrodeassembly, at least one of the cathode catalyst layer and the anodecatalyst layer is the catalyst layer according to the embodimentdescribed above.

However, by taking into consideration necessity of improved protonconductivity and improved transport characteristic (gas diffusibility)of a reaction gas (particularly, O₂), at least the cathode catalystlayer is preferably the catalyst layer according to the embodimentdescribed above. However, the catalyst layer according to the embodimentis not particularly limited. The catalyst layer may be used as the anodecatalyst layer or may be used as the cathode catalyst layer and theanode catalyst layer.

According to further embodiment of the present invention, provided is afuel cell including the membrane electrode assembly according to theembodiment. Namely, according to one aspect, the present inventionprovides a fuel cell comprising a pair of anode separator and cathodeseparator interposing the membrane electrode assembly according to theembodiment.

Hereinafter, members of a PEFC 1 using the catalyst layer according tothe embodiment will be described with reference to FIG. 1. However, thepresent invention has features with respect to the catalyst layer.Therefore, among members constituting the fuel cell, specific forms ofmembers other than the catalyst layer may be appropriately modified withreference to well-known knowledge in the art.

(Electrolyte Membrane)

An electrolyte membrane is configured with a solid polymer electrolytemembrane 2 in the same form illustrated in, for example, FIG. 1. Thesolid polymer electrolyte membrane 2 serves to selectively transmitprotons generated in an anode catalyst layer 3 a to a cathode catalystlayer 3 c in the thickness direction during the operation of the PEFC 1.In addition, the solid polymer electrolyte membrane 2 also serves as apartition wall for preventing a fuel gas supplied to an anode side frombeing mixed with an oxidant gas supplied to a cathode side.

An electrolyte material constituting the solid polymer electrolytemembrane 2 is not particularly limited, but well-known knowledge in theart may be appropriately referred to. For example, the fluorine-basedpolymer electrolyte or the hydrocarbon-based polymer electrolytedescribed above as the polymer electrolyte can be used. There is no needto use the polymer electrolyte which is necessarily the same as thepolymer electrolyte used for the catalyst layer.

A thickness of the electrolyte layer is not particularly limited, but itmay be determined by taking into consideration characteristics of theobtained fuel cell. The thickness of the electrolyte layer is typicallyin the range of about 5 to 300 μm. If the thickness of the electrolytelayer is within such a range, balance between strength during the filmformation or durability during the use and output characteristics duringthe use can be appropriately controlled.

(Gas Diffusion Layer)

A gas diffusion layer (anode gas diffusion layer 4 a, cathode gasdiffusion layer 4 c) serves to facilitate diffusion of a gas (fuel gasor oxidant gas) supplied through a gas passage (6 a, 6 c) of a separatorto a catalyst layer (3 a, 3 c) and also serves as an electron conductingpath.

A material constituting a substrate of the gas diffusion layers (4 a, 4c) is not particularly limited, but well-known knowledge in the relatedart may be appropriately referred to. For example, a sheet-shapedmaterial having conductivity and porous property such as a fabric madeof carbon, a sheet-shaped paper, felt, and a nonwoven fabric may beexemplified. A thickness of the substrate may be appropriatelydetermined by considering characteristics of the obtained gas diffusionlayer. The thickness of the substrate may be in the range of about 30 to500 μm. If the thickness of the substrate is within such a range,balance between mechanical strength and diffusibility of gas, water, andthe like can be appropriately controlled.

The gas diffusion layer preferably includes a water repellent for thepurpose of preventing a flooding phenomenon or the like by improvingwater repellent property. The water repellent is not particularlylimited, but fluorine-based polymer materials such aspolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF),polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylenecopolymer (FEP), polypropylene, polyethylene, and the like may beexemplified.

In order to further improve water repellent property, the gas diffusionlayer may include a carbon particle layer (microporous layer (MPL), notshown) configured with an assembly of carbon particles including a waterrepellent provided at the catalyst-layer side of the substrate.

Carbon particles included in the carbon particle layer are notparticularly limited, but well-known materials in the art such as carbonblack, graphite, and expandable graphite may be appropriately employed.Among the materials, due to excellent electron conductivity and a largespecific surface area, carbon black such as oil furnace black, channelblack, lamp black, thermal black, and acetylene black can be preferablyused. An average particle diameter of the carbon particles may be set tobe in the range of about 10 to 100 nm. By this, high water-repellentproperty by a capillary force can be obtained, and contacting propertywith the catalyst layer can be improved.

As the water repellent used for the carbon particle layer, theabove-described water repellent may be exemplified. Among the materials,due to excellent water repellent property and excellent corrosionresistance during the electrode reaction, the fluorine-based polymermaterial can be preferably used.

A mixing ratio of the carbon particles and the water repellent in thecarbon particle layer may be set to be in the range of weight ratio ofabout 90:10 to 40:60 (carbon particle: water repellent) by taking intoconsideration balance between water repellent property and electronconductivity. Meanwhile, a thickness of the carbon particle layer is notparticularly limited, but it may be appropriately determined by takinginto consideration water repellent property of the obtained gasdiffusion layer.

(Method of Manufacturing Membrane Electrode Assembly)

A method of manufacturing a membrane electrode assembly is notparticularly limited, and a well-known method in the art maybe used. Forexample, a method which comprises transferring a catalyst layer to asolid polymer electrolyte membrane by using a hot press, or coating asolid polymer electrolyte membrane with a catalyst layer and drying thecoating, and joining the resulting laminate with gas diffusion layers,or a method which comprises coating a microporous layer (in the case ofnot including a microporous layer, one surface of a substrate layer) ofa gas diffusion layer with a catalyst layer in advance and drying theresulting product to produce two gas diffusion electrodes (GDEs), andjoining both surfaces of the solid polymer electrolyte membrane with thetwo gas diffusion electrodes by using a hot press can be used. Thecoating and joining conditions by hot press and the like may beappropriately adjusted according to a type of the polymer electrolyte(perfluorosulfonic acid-based or hydrocarbon-based) in the solid polymerelectrolyte membrane or the catalyst layer.

(Separator)

In the case of configuring a fuel cell stack by connecting a pluralityof unit fuel cells of polymer electrolyte fuel cells in series, aseparator serves to electrically connect the cells in series. Theseparator also serves as a partition wall for separating a fuel gas, anoxidant gas, and a coolant from each other. In order to secure a passagethereof, as described above, gas passages and coolant passages arepreferably installed in each of the separators. As a materialconstituting the separator, well-known materials in the art of carbonsuch as dense carbon graphite and a carbon plate, a metal such as astainless steel, or the like can be employed without limitation. Athickness or size of the separator, a shape or size of the installedpassages, and the like are not particularly limited, but they can beappropriately determined by taking into consideration desired outputcharacteristics and the like of the obtained fuel cell.

A manufacturing method for the fuel cell is not particularly limited,and well-known knowledge in the art in the field of fuel cell may beappropriately referred to.

Furthermore, in order that the fuel cell can generate a desired voltage,a fuel cell stack may be formed by connecting a plurality of membraneelectrode assemblies in series through a separator. A shape and the likeof the fuel cell are not particularly limited, and they may beappropriately determined so as to obtain desired cell characteristicssuch as a voltage.

The above-described PEFC or membrane electrode assembly uses thecatalyst layer having excellent power generation performance andexcellent durability. Therefore, the PEFC or membrane electrode assemblyshows excellent power generation performance and durability.

The PEFC according to the embodiment and the fuel cell stack using thePEFC can be mounted on a vehicle, for example, as a driving powersource.

EXAMPLE

The effects of the present invention will be described with reference tothe following Examples and Comparative Examples. However, the scope ofthe present invention is not limited to the Examples.

Example 1

A carbon material A which had a pore volume of micropores of 1.04 cc/g;a pore volume of mesopores of 0.92 cc/g; a mode diameter of microporesof 0.65 nm; a mode diameter of mesopores of 1.2 nm; and a BET specificsurface area of 1770 m²/g was prepared as below. Specifically, thecarbon material A was prepared by a method disclosed in WO 2009/75264 A.

Next, the resultant carbon material A was heated to 1700° C. at atemperature rising rate of 500° C./h under an argon atmosphere and wasthen held at this temperature for five minutes, to prepare a support Ahaving a BET specific surface area of 1378 m²/g. An R value and an R′value of the support A were measured to be 1.99 and 0.42, respectively.In addition, the resultant support A was measure for an average particlesize (diameter), pore volumes of micropores and mesopores, modediameters of micropores and mesopores, and a BET specific surface area.As a result, it was found that for the support A, an average particlesize (diameter) was 91.5 nm, a pore volume of micropores was 0.43cc/g-support, a pore volume of mesopores was 0.69 cc/g-support, a modediameter of the micropores was 0.66 nm, a mode diameter of the mesoporeswas 2.8 nm, and a BET specific surface area was 1378 m²/g.

Example 2

A carbon material A was prepared in the same manner as in SynthesisExample 1.

Next, the resultant carbon material A was heated to 1600° C. at atemperature rising rate of 500° C./h under an argon atmosphere and wasthen held at this temperature for five minutes, to prepare a support Bhaving a BET specific surface area of 1522 m²/g. An R value and an R′value of the support B were measured to be 1.81 and 0.50, respectively.In addition, the resultant support B was measure for an average particlesize (diameter), pore volumes of micropores and mesopores, modediameters of micropores and mesopores, and a BET specific surface area.As a result, it was found that for the support B, an average particlesize (diameter) was 89 nm, a pore volume of micropores was 0.73cc/g-support, a pore volume of mesopores was 1.17 cc/g-support, a modediameter of the micropores was 0.73 nm, a mode diameter of the mesoporeswas 2.4 nm, and a BET specific surface area was 1522 m²/g.

Comparative Example 1

A carbon material A was prepared in the same manner as in SynthesisExample 1.

Next, an R value and an R′ value of a support C using the carbonmaterial A were measured to be 1.64 and 0.61, respectively. In addition,the resultant support C was measure for an average particle size(diameter), pore volumes of micropores and mesopores, mode diameters ofmicropores and mesopores, and a BET specific surface area. As a result,it was found that for the support C, an average particle size (diameter)was 91.5 nm, a pore volume of micropores was 1.04 cc/g-support, a porevolume of mesopores was 1.23 cc/g-support, a mode diameter of themicropores was 0.65 nm, a mode diameter of the mesopores was 2.1 nm, anda BET specific surface area was 1768 m²/g.

Comparative Example 2

A carbon material A was prepared in the same manner as in SynthesisExample 1.

Next, the resultant carbon material A was heated to 1300° C. at atemperature rising rate of 500° C./h under an argon atmosphere and wasthen held at this temperature for five minutes, to prepare a support D.An R value and an R′ value of the support D were measured to be 1.75 and0.66, respectively. In addition, the resultant support D was measure foran average particle size (diameter), pore volumes of micropores andmesopores, mode diameters of micropores and mesopores, and a BETspecific surface area were measured. As a result, it was found that forthe support D, an average particle size (diameter) was 91.5 nm, a porevolume of micropores was 1.06 cc/g-support, a pore volume of mesoporeswas 1.21 cc/g-support, a mode diameter of the micropores was 0.66 nm, amode diameter of the mesopores was 2.1 nm, and a BET specific surfacearea was 1768 m²/g.

Comparative Example 3

Ketjen Black (EC300J) (having a BET specific surface area of 715 m²/g)was used as a support E. An R value and an R′ value of the support Ewere measured to be 1.78 and 0.74, respectively. In addition, thesupport resultant E was measure for an average particle size (diameter),pore volumes of micropores and mesopores, mode diameters of microporesand mesopores, and a BET specific surface area. As a result, it wasfound that for the support E, an average particle size (diameter) was 53nm, a pore volume of micropores was 0.35 cc/g-support, a pore volume ofmesopores was 0.49 cc/g-support, a mode diameter of the micropores was0.45 nm, a mode diameter of the mesopores was 2.2 nm, and a BET specificsurface area was 715 m²/g.

Experiment 1: Evaluation of Platinum Coating

A platinum specific surface area (COMSA) was measured using COadsorption method when platinum was supported on the supports A and B ofthe present invention produced in Examples 1 and 2 described above andthe support E produced in Comparative Example 3 at a ratio of 50 wt %relative to the weight of the support. The result is illustrated in FIG.4.

It is noted from FIG. 4 that the supports (carbon powders) A and Bhaving BET specific surface area of 900 m²/g or more have asignificantly larger platinum specific surface area as compared to thatof the support E having BET specific surface area deviated from therange of the present invention. From this, it is considered that theelectrical double layer capacitance of the catalyst can be significantlyimproved by using the carbon powder of the present invention as asupport.

Example 3

The support A manufactured in the Example 1 was used, and platinum (Pt)having an average particle diameter of more than 3 nm and 5 nm or lessas a catalyst metal was supported on the support A at a support ratio of30 wt %, to prepare a catalyst powder A. To be specific, 46 g of thesupport A was immersed into 1000 g of a dinitrodiammine platinum nitricacid solution having a platinum concentration of 4.6 wt % (platinumcontent: 46 g), and after stirring, 100 mL of 100% of ethanol as areducing agent was added thereto. The resultant mixture was stirred andmixed at a boiling point for 7 hours, so that platinum was supported onthe support A. Next, by filtering and drying, the catalyst powder havinga support ratio of 30 wt % was obtained. After that, the resultingproduct was maintained in a hydrogen atmosphere at a temperature of 900°C. for 1 hour, to yield a catalyst powder A.

Example 4

A catalyst powder B was obtained by the same processes as those ofExample 3, except that the support B manufactured in the Example 2 wasused instead of the support A in Example 3.

Comparative Example 4

A catalyst powder C was obtained by the same processes as those ofExample 3, except that the support C manufactured in the ComparativeExample 1 was used instead of the support A in Example 3.

Comparative Example 5

A catalyst powder D was obtained by the same processes as those ofExample 3, except that the support D manufactured in the ComparativeExample 2 was used instead of the support A in Example 3.

Experiment 2: Evaluation of Durability

The catalyst powders A and B produced in Examples 3 and 4 and thecatalyst powders C and D produced in Comparative Examples 4 and 5 wereevaluated for durability according to the following method. The resultsare indicated in Table 1 below.

To be specific, using a three-electrode type electrochemical cell, anelectrochemical system “HZ-5000+HR301” manufactured by a Hokuto DenkoCorporation was used as a potentiostat. By using a glassy carbon rotaryelectrode (GC-RDE) (φ (diameter)=5 mm) as a working electrode, an inkobtained by dispersing each catalyst powder produced in Examples andComparative Examples in a mixed solvent of water and 1-propanol servingas a dispersion medium was coated thereon and dried so as to give a dryfilm thickness of 1 μm, to produce an electrode. Carbon was used as acounter electrode, and a reversible hydrogen electrode (RHE) was used asreference electrode. 0.1 M of perchloric acid as an electrolyte wassaturated with O₂. The measurement was performed at 60° C.

An effective catalyst surface area (ECA) was calculated by a cyclicvoltammetry (CV). Potential scanning was performed at 1.0 V for 30seconds before measurement. Thereafter, the potential range of from 1.0to 1.5 V rose (for one second) and descended (for one second) at apotential sweep rate of 0.5 V/s, and this process was referred to as onecycle (2 seconds/cycle). When this potential cycle was repeated, a peakpotential of a quinine-hydroquinone reduction current in the vicinity of0.6 V measured by a cyclic voltammetry method was shifted to a lowpotential side with an increase in the potential cycle. From the changein reduction current, a carbon state and a change of the electricaldouble layer capacitance were estimated. Specifically, the number ofcycles to be repeated until the potential of the reduction currentbecame 0.5 V or less was used as an index of durability.

TABLE 1 Number of cycles until peak Catalyst R′ R potential of reductioncurrent powder value value becomes 0.5 V or less Example 3 A 0.42 1.993766 Example 4 B 0.50 1.81 1724 Comparative C 0.61 1.64 1350 Example 4Comparative D 0.66 1.75 778 Example 5

It is noted from the results in Table 1 that the numbers of cycles todecrease the reduction current appeared in the catalyst powders A and Bprepared in Examples 3 and 4 are larger than those appeared in thecatalyst powders C and D prepared in Comparative Examples 4 and 5. Fromthis, it is considered that the catalyst using the carbon powder of thepresent invention has small decrease in electrical double layercapacitance, and can maintain significantly high activity (excellent indurability).

Example 5

A catalyst powder E was obtained by the same processes as those ofExample 3, except that platinum (Pt) was supported on the support A at asupport ratio of 50 wt % in Example 3.

Comparative Example 6

A catalyst powder F was obtained by the same processes as those ofExample 5, except that the support C manufactured in the ComparativeExample 1 was used instead of the support A in Example 5.

Example 6

The catalyst powder E manufactured in Example 5 and an ionomerdispersion liquid (Nafion (registered trademark) D2020, EW=1100 g/mol,produced by DuPont) as a polymer electrolyte were mixed at a weightratio of the carbon support and the ionomer of 0.9. Next, a cathodecatalyst ink was prepared by adding a n-propyl alcohol solution (50%) asa solvent with a solid content (Pt+carbon support+ionomer) of 7 wt %.

Ketjen Black (particle diameter: 30 to 60 nm) was used as the support,and platinum (Pt) having an average particle diameter of 2.5 nm as thecatalyst metal was supported thereon at a support ratio of 50 wt %, toobtain a catalyst powder. The catalyst powder and an ionomer dispersionliquid (Nafion (registered trademark) D2020, EW=1100 g/mol, produced byDuPont) as the polymer electrolyte were mixed at a weight ratio of thecarbon support and the ionomer of 0.9. Next, an anode catalyst ink wasprepared by adding a n-propyl alcohol solution (50%) as a solvent with asolid content (Pt+carbon support+ionomer) of 7 wt %.

Next, a gasket (Teonex produced by Teijin DuPont, thickness: 25 μm(adhesive layer: 10 μm)) was arranged around both surfaces of a polymerelectrolyte membrane (NAFION NR211 produced by DuPont, thickness: 25μm). Then, an exposed portion of one surface of the polymer electrolytemembrane was coated with the cathode catalyst ink having a size of 5cm×2 cm by a spray coating method. The catalyst ink was dried bymaintaining the stage where the spray coating was performed at atemperature of 60° C. for 1 minute, to obtain a cathode catalyst layer.At this time, a supported amount of platinum is 0.15 mg/cm². Next,similarly to the cathode catalyst layer, an anode catalyst layer wasformed by spray coating and heat-treatment on the electrolyte membrane.

The resultant laminate were sandwiched by gas diffusion layers (24BC,manufactured by SGL Group-The Carbon Company), to obtain a membraneelectrode assembly (1) (MEA (1)).

Comparative Example 7

A membrane electrode assembly (2) (MEA (2)) was manufactured by the sameprocesses as those of Example 6, except that the catalyst powder Fobtained in Comparative Example 6 was used instead of the catalystpowder E in Example 6.

Experiment 3: Evaluation of Power Generation Performance

A single cell of a fuel cell was configured using the MEA (1) producedin Example 6 described above and the MEA (2) produced in ComparativeExample 7, respectively, and evaluated for power generation performance(durability) according to the following method. The result isillustrated in FIG. 5.

First, the single cell of the fuel cell was controlled at a temperatureof 80° C., a hydrogen gas and nitrogen were supplied to an anode sideand a cathode side of the fuel cell, respectively, and an exhaust-sidepressure of the fuel cell was set to be an atmospheric pressure.

An external load was controlled for three seconds in this state at anoperation voltage of the single cell of 0.6 V, and then the externalload was controlled for three seconds at an operation voltage of 0.9 V.

Then, the single cell was operated by the same control of operationtemperature and operation voltage of the single cell was repeated with1000 cycles.

Before and after the load cycle operation of the fuel cell, anelectrochemical surface area (ECA) of a cathode catalyst layer wascalculated from an area of a reduction current corresponding to hydrogengeneration measured by a cyclic voltammetry method. By setting aninitial electrochemical surface area to be 1, decrease inelectrochemical surface area due to potential cycles was determined, toevaluate durability of the fuel cell depending on a change in theeffective surface area.

It is noted from the result illustrated in FIG. 5 that the MEA (1) ofExample 6 has smaller decrease in in electrochemical surface area thanthat of the MEA (2) of Comparative Example 7. From this, it isconsidered that the membrane electrode assembly using the catalystformed using the carbon powder of the present invention can exhibit andmaintain high power generation performance.

The present application is based on the Japanese Patent application No.2013-204163 filed on Sep. 30, 2013, the entire disclosed contents ofwhich are incorporated herein by reference.

1. A carbon powder for catalyst containing as a main component carbon,which has a BET specific surface area per unit weight of 900 m²/g orgreater, and a ratio R′ (D′/G intensity ratio) of peak intensity for aD′-band (D′ intensity) measured in the vicinity of 1620 cm⁻¹ to peakintensity for a G-band (G intensity) measured in the vicinity of 1580cm⁻¹ by Raman spectroscopy of 0.6 or less.
 2. The carbon powder forcatalyst according to claim 1, which has a ratio R (D/G intensity ratio)of peak intensity for a D-band (D intensity) measured in the vicinity of1360 cm⁻¹ to the peak intensity for the G-band (G intensity) measured inthe vicinity of 1580 cm⁻¹ by Raman spectroscopy of 1.7 or more.
 3. Acatalyst having a catalyst metal supported on the carbon powder forcatalyst set forth in claim
 1. 4. The catalyst according to claim 3,wherein the catalyst metal is platinum or comprises platinum and metalcomponents other than platinum.
 5. An electrode catalyst layer for fuelcell comprising the catalyst set forth in claim 3 and an electrolyte. 6.A membrane electrode assembly for fuel cell comprising the electrodecatalyst layer for fuel cell set forth in claim
 5. 7. A fuel cellcomprising the membrane electrode assembly for fuel cell set forth inclaim
 6. 8. A catalyst having a catalyst metal supported on the carbonpowder for catalyst set forth in claim
 2. 9. An electrode catalyst layerfor fuel cell comprising the catalyst set forth in claim 4 and anelectrolyte.
 10. The catalyst according to claim 8, wherein the catalystmetal is platinum or comprises platinum and metal components other thanplatinum.